This invention relates to method of controlling the propagation characteristics of radiation in waveguides by means of photonic band gaps, to optical devices and, in particular, to optical devices which influence the transmission of radiation by means of photonic band gaps. Such devices may be formed by etching a substance which supports propagation of radiation at a wavelength of interest. Although the embodiments described herein are concerned with visible radiation, the principles involved are equally applicable to techniques for controlling the propagation of other forms of electromagnetic radiation such as ultra-violet, infra-red, terahertz and microwave radiation. In this specification, the term xe2x80x9copticalxe2x80x9d includes such other forms of radiation.
For some periodic dielectric structures, the propagation of electromagnetic radiation can become forbidden in certain lattice directions. These structures are known as photonic band gap structures. Structures based upon a cubic or triangular lattice of deep air rods in a background dielectric material can exhibit a photonic band gap (PBG). The size and position of the band gap is dependent upon the wave polarisation state, direction of wave propagation, dimensions of the photonic crystal, and the dielectric contrast. The frequency extent of the band gap is of the order of the lattice spacing. Semiconductor materials are ideal for the fabrication of PBGs because of their large dielectric constant. It has also been shown that two-dimensional photonic lattices can have a three dimensional band gap, that is to say, the band gap remains open even when there is a large out of plane wave component.
Photonic band structures with band gaps at optical frequencies have several interesting applications. An important property of photonic band gaps is the ability to enhance or inhibit spontaneous emission within the band gap energy range. This has important implications for direct band gap optoelectronic devices such as semiconductor lasers and light-emitting diodes (LEDs).
Photonic band gap structures can also be fabricated in fluorescent (including laser) materials. The PBG can make these active materials useful as sensors, or to make one transition (or group of transitions) more likely to occur than others.
As a sensor, the PBG may be fabricated to fluoresce at a specified wavelength when the air holes in the structure are filled with air. If however the air holes fill with a different gas, such as pure carbon dioxide, or carbon monoxide, the different refractive index of the gas (compared to ordinary air) could be made to tune the PBG off the fluorescent line which would be easily detected. The PBG structure may be used in a similar way for liquid sensing.
Some laser glasses emit at several different wavelengths (example neodymium-doped GLS glass). Frequently, it is desirable to choose preferentially to amplify just one line. This line may be the weakest transition of the group of lines. A PBG structure in the glass may be employed to prevent the fluorescence of the unwanted lines and promote the transmission of the required wavelength.
A particularly important application would be to make a high energy laser transition in a glass favourable by preventing direct transitions from lower lying radiative levels. In a typical laser system, the lower lying transitions are stronger and more likely to occur. However, there may be useful higher energy levels (for example in the blue region of the spectrum) that could be used, but that are unusable because the lower energy transitions are taking all the energy. A suitably engineered PBG in such a laser system could prevent the lower energy transitions from occurring, thus allowing lasing at the higher energy level.
PCT patent application No. WO 94/16345 (Massachusetts Institute of Technology) discloses low-loss optical and opto-electronic integrated circuits with light guides fabricated in a structure having a photonic band gap. This publication does not disclose methods for the determination of the transmission characteristics of such waveguides, other than the centre frequency of the band gap. Furthermore, it describes embodiments which would not operate in the manner described therein, due to adverse interaction between a photorlic band gap and a dielectric waveguide. Another disclosed embodiment would not produce the promised advantage due to the influence of back reflection in a tapered dielectric waveguide.
An etched silicon structure is disclosed by V. Lehmann in J. Electrochem. Soc. Vol. 140, No. 10, page 2836, October 1993. However, use of the etched silicon structure as an optical device is not discussed. The etched silicon structure disclosed is formed by etching an homogeneous slab of bulk silicon by placing it in an acid bath. Etching is achieved by establishing an electric field across two opposite, substantially planar, faces of the silicon slab, and illuminating the rear surface. The resultant structure has an array of substantially equally spaced holes or pores formed therein. These holes or pores are referred to as macro pores and occur as a result of an electro-chemical reaction in conjunction with the phenomenon of a self adjusting charge distribution at a tip of a macro pore.
Krauss T. F. et al., in Nature 1996 (Oct. 24, 1996) Vol 383 at pages 699-702, describe a photonic bandgap (PBG) device. The device is a two-dimensional lattice in the form of an homogeneous array of holes formed in a semiconductor waveguide of high refractive index silicon. Krauss notes that radiation from a tunable source, incident on the structure at certain angles, is detected as it emerges from a waveguide positioned on a substantially opposite side to where the radiation is incident.
According to the present invention there is provided an optical device including a waveguide formed in a first region of a first optically-transmissive material bounded by a second region or regions having an array of sub-regions arranged therein to create a photonic bandgap at least partially non-transmissive to radiation of a predetermined frequency or frequencies wherein the frequency transmission characteristics of said waveguide are at least partially determined by the transmission characteristics of said second region or regions.
There is also provided an optical transfer device having a first plurality of input ports and a second plurality of output ports coupled by a waveguide at least partially bounded by a photonic band gap wherein at least one of said first plurality of ports is adapted to pass an optical signal having a first range of frequencies and at least one of said second plurality of ports is adapted to pas an optical signal having a second range of frequencies, said first and second range of frequencies being defined by said photonic band gap.
There is also provided an active optical device having a waveguide comprising a region of optically-transmissive material bounded by a photonic band gap and containing a dopant adapted to induce quasi-stable energy levels in said material.
The invention further provides a hybrid opto-electronic signal translation device having a first region adapted to transfer a signal by means of movement of electrical charge carriers and a second region adapted to transfer a corresponding signal by means of electromagnetic radiation and electro-optic transducer means disposed between said first and second regions to convert said signal from or to said corresponding signal wherein said second region includes a third region at least partially bounded by a photonic band gap.
The invention further provides a coupler to a waveguide defined by a photonic band gap having an input or output port for the transfer of radiation to or from said waveguide wherein said input or output port includes a region having a graded refractive index to enhance the transfer of radiation to or from said waveguide.
There is also provided a method of fabricating an optical device comprising the steps of forming a waveguide in a first region of a first optically-transmissive material by creating in a second region or regions an array of sub-regions having a photonic bandgap at least partially non-transmissive to radiation of a predetermined frequency or frequencies wherein the radiation transmission characteristics of said waveguide are at least partially determined by the transmission characteristics of said second region or regions.
According to an aspect of the present invention there is provided an optical device comprising a substrate supporting a waveguide, an input channel and at least two output channels in optical connection with said waveguide, the waveguide being formed from a material of a first refractive index and having an array of regions formed therein, the regions having a different refractive index to that of the waveguide, so that a beam of radiation incident on the device is split into at least two output beams.
Preferably, the intensities of the output beams are substantially equal.
According to a particular aspect of the present invention, the optical device may be used as a WDDM. The WDDM may be used as a wavelength multiplexer.
According to a particular aspect of the present invention, the optical device is adapted to separate a group of information channels from a plurality of input channels encoded by wavelength.
According to another aspect of the present invention there is provided a device comprising an etched semiconductor substrate, characterised in that a plurality of holes or perforations are formed in the substrate the holes or perforations are of a non-uniform nature and/or have a non-uniform inter-hole spacing.
According to another aspect of the invention, there is provided a method of manufacturing an optical device comprising the steps of forming a waveguide on a substrate the waveguide having a first refractive index and forming an array of regions in the waveguide, said regions having a different refractive index to the waveguide.
Preferably the optical device is formed from an etched semiconductor comprising a silicon substrate and at least one overlying layer.
According to another aspect of the invention there is provided a method of multiplexing, or demultiplexing, a plurality of signals comprising combining, or splitting, said signals using an optical device as hereinbefore described.
According to a yet further aspect of the invention there is provided a method of exposing electromagnetic signal(s) to an array of regions having a first dielectric constant disposed within a waveguide formed from a second dielectric constant and varying at least one of said dielectric constants so as to vary a characteristic of said signal(s).
An example of an optical device which may advantageously be made in accordance with one aspect of the present invention is a wavelength division de-multiplexer (WDDM). A WDDM splits a single incident beam of radiation, carrying data, into two or more beams of different wavelength. Each of the split beams carries different data from that carried by another beam. Wavelengths are selected such that data transmitted at one wavelength does not interfere with data transmitted at another wavelength. The result is that one data channel, such as an optical fibre, is capable of carrying several different data signals encoded by the wavelength of the carrier signal. Thus, the data carrying capacity of the fibre is increased.
Prior WDDM""s suffer from the disadvantage that the minimum bandwidth of a channel can be large. Also these devices are discrete components, are difficult to align, and are not robust. They are also polarisation insensitive.
A wavelength multiplexer is a device which transfers a Wavelength Division Multiplexer (WDM) encoded input signal to a plurality of output channels, whilst simultaneously routing a selected group of wavelength signals to a predetermined group of output channels. A wavelength multiplexer which includes the optical device of the present invention therefore has the added feature of wavelength selectivity.
A splitter separates a single incident beam of radiation carrying data, into two or more beams, of reduced power. Each of the split beams carries identical information. The result is that a single data channel can be distributed to several different destinations simultaneously. An input channel may consist of a plurality of data channels, each encoded according to the wavelength of a carrier signal, or by Time Division Multiplexing (TDM). In such an arrangement all data signals input to the device from the input data channel are routed simultaneously to all of the output channels.
Wavelength Division Multiplexers (WDM) are able selectively and simultaneously to route a given wavelength encoded input data channel to a pre-defined sub-group of output channels. A sub-group of output channels may be different for each wavelength encoded input data channel. In addition, the sub-group of output channels may be further reduced or increased according to the electromagnetic polarisation state of the original input data channel.
The input channel may also be encoded by electromagnetic polarisation state. This doubles the capacity of the input channel. However, prior grating beam splitters or multiplexers suffer from the disadvantage that beams split from a single channel may have widely varying intensities or powers. Another problem suffered by existing splitters and multiplexers is that the maximum number of output channels is also quite small. The present invention overcomes these problems, providing an optical device suitable for use with data carrying channels.
An example of an optical device in accordance with another aspect of the present invention is an optical signal cross-connect. A cross connect allows simultaneous bi-directional communication between a plurality of data channels so that data signals input to the device from any single channel are distributed simultaneously to all other channels. Input channels may carry data signals encoded by carrier wavelength, electromagnetic polarisation state, or by Time Division Multiplexing (TDM). A single device then allows simultaneous bi-directional communication between several sets of transceivers, maintaining high channel separation between them all. Present cross connects may be xe2x80x98mode dependentxe2x80x99 resulting in a significant variation in power between the output channels. In addition wavelength selectivity can be incorporated in to this exchange to route a group of wavelength signals to a group of destinations.
Preferably the array of regions formed in the waveguide is in the form of regular hexagonal pattern with the axes of holes being orthogonal to the surface of the waveguide. In this arrangement a single input beam may be split into a plurality of output beams.
Preferably the input beam is split into six output beams.
The optical device may be used as part of a combiner, in which case a plurality of input beams incident on the device may be combined into a single output beam.
Preferably the depth of the waveguide is substantially constant. The array may be disposed in three-dimensional pattern, throughout the volume of the waveguide.
An optical device which may be made in accordance with yet another aspect of the present invention is an integrated optical polarisation controller. A randomly polarised input beam of arbitrary wavelength is separated into TE and TM polarisation states. The invention may be used with such a device, so that a sub-group of channels may be polarisation multiplexed as well as multiplexed as described above.
If a defect, such as a line defect, is introduced into the device, sharp bends may be created into an optical path in an integrated planar waveguide. At present this is impossible to achieve by other methods.
Yet further optical devices may be used as part of a photonic band pass filter. In such an arrangement, the inclusion of xe2x80x98defectsxe2x80x99 within the otherwise periodic lattice improves the performance of the device, creating a narrow passband within the wavelength range of a stopband.
The array may be in the form of another shape such as, for example, a square or it may be xe2x80x98quasi periodicxe2x80x99. This gives a different number of output beams (e.g. 4 or 2 for a square lattice). By quasi-periodic, in this instance, what is meant is a structure which may be composed of a superposition of two regular lattices which then result in a non uniform lattice. It may also be in the form of a lattice the spacing and/or packing arrangement of which changes along a given dimension in a predetermined manner.
According to another aspect of the present invention there is provided a device comprising an etched semiconductor substrate, characterised in that a plurality of holes or perforations are formed in the substrate the holes or perforations are of a non-uniform nature and/or have a non-uniform inter-hole spacing.
The etching technique has made possible the fabrication of waveguide beam splitters and 90xc2x0 bends. The bend radius (with zero loss) is xcx9c50 xcexcm as opposed to the current state of the art xcx9c10 mm using other techniques. The possible limit for 633 nm radiation is xcx9c2 xcexcm. This makes feasible an optical interconnect in chip-scale integration for computing and communications applications.
The holes or perforations are disposed in an array through a semiconductor substrate so that the inter-hole spacing varies in a predetermined manner.
In accordance with one aspect of the invention, the variation in the inter-hole spacing or diameter of holes is such that a physical characteristic of radiation incident on the optical device is varied. Thus, for example, between first and second adjacent rows, along an edge of the array, the inter-hole or perforation spacing may be 10 xcexcm; and between a second and a third row of holes in the array, the inter-hole spacing may be 100 xcexcm.
The spacing between adjacent rows may increase by a regular amount from one pair of adjacent rows to the next. Holes or perforations may be grouped into rows and columns, or may be arranged in a circular, triangular, square, spiral or any other shaped pattern.
This variation in inter-row (or column) spacing of an array may increase linearly or non-linearly. For example an inter-hole or perforation spacing may be defined as xe2x80x98dxe2x80x99 and a relationship between spacing of adjacent rows I may be expressed as In+1=In+kd where k is any positive number. This simple linear relationship is discussed in greater detail below. It will be appreciated that the spacing may increase non-linearly.
For WDDM, most appropriate defects are where the relative diameter of a fraction of the holes is increased or reduced.
The set of defects may be arranged in a regular fashion or perhaps superimposed randomly throughout a regular lattice. The quantity of defective holes may determine the efficiency of the effect.
A medium of variable refractive index may be disposed in the holes or perforations. Means may be provided for varying the refractive index of the medium. Additionally, nonlinearity may be introduced by the presence of dopants, creating quasi-stable energy levels which absorb or emit radiation.
The refractive index of the medium in the holes or perforations may be varied by exposing the medium to an electric or magnetic field which is changed by way of a controller. Such a modified device provides a selectively variable optical switch. Alternatively the medium of variable refractive index may comprise a multi-layered structure formed by etching or crystal growth.
Photonic band gap structures can be fabricated in fluorescent (including laser) materials. The PBG can make these active materials useful as sensors, or to make one transition (or group of transitions) more likely to occur than others.
As a sensor, the PBG may be fabricated to fluoresce at a specified wavelength when the air-holes in the structure are filled with air. If however the air holes fill with a different gas, such as pure carbon dioxide, or carbon monoxide, the different refractive index of the gas (compared to air) could be made to tune the PBG off the fluorescent line which would be easily detected. The PBG structure could be used in a similar way for liquid sensing.
Some laser glasses emit at several different wavelengths (example neodymium-doped GLS glass) and we may want to choose to preferentially amplify just one line, and often that line is the weakest transition of the group of lines. A PBG structure in the glass can prevent the fluorescence of the unwanted lines and promote the transmission of the required wavelength.
A particularly important application would be to make a high energy laser transition in a glass favourable by preventing direct transitions from lower lying radiative levels. In a typical laser system, the lower lying transitions are stronger and more likely to occur. However, there may be useful higher energy levels (for example in the blue region of the spectrum) that could be used, but that are unusable because the lower energy transitions are taking all the energy. A properly engineered PBG in such a laser system could prevent the lower energy transitions from occurring, thus allowing lasing at the higher energy level.
Photons lying near the edge of the band gap in energy will be considerably reduced in velocity through the PBG structure (within the band gap itself they stop, they are standing waves). By fabricating a PBG region which is close to the transmitted (information carrying) photon energy, the photon stream can be slowed downxe2x80x94the wave velocity is reduced. This would allow signal processing of the data to occur in more reasonable time scales (in exactly the same way that delay lines are used in signal processing electrical signals).